Hi all,=20
I'm experimenting in this simple circuit on the bredboard (from the =
LM358 Datasheet, see below).
Main idea was to build an inexpensive sensor to check water =
conductivity, simply with the substitution of  R1 with two electrodes =
and subsequently to measure the output frequency with TMR0 ( PIC 16F84 / =
16F628 ).

I imagined to obtain, with differents values of R1 (and a fixed value of =
C1) a linear variation of frequency, at least within an 1Khz - 20Khz =
interval but....the values don't seems very good ! =20
In the doubt, I have tested another similar circuit (but with a LM393 =
comparator): the absolute values are partially differents but =
proportionals with these results.=20

Well, I'm not an electronic expert and probably I'm forgetting =
something...but what?? =20
Is it correct this operation?

For reference, these are the values recorded with the multimeter and PIC =
TMR0:

( C =3D 47nF, ceramic) -  Supply =3D ~ 5.2V


R1 (Ohm) Freq.(Hz)  PIC TMR0
1000     745        94
2700    1092       135
4700    2220       274
10000   3500       438
15000   7360       923


                         R1 (probe)
                        _____
                   +---|_____|---------+=20
           C 47nF  |                   |
               ||  |    |\ -           |            ____     ____
         +-----||-------| \  1/2 LM358 |           |    |   |    |  =20
         |     ||       |  \_____ _____|__  Vo   __|    |___|    |__
    GND  |         +----|  /           |
        ---        |    | /            |
                   |    |/ +           |
          _____    |    _____          |
  V+  ---|_____|---+---|_____|---------+
                   |
       R2 100K    --.   R4 100K
                 |  |=20
                 |  |
                 |  |  R3 100K
                  --
                   |
             GND   |
                 --+--              =20


Any help will be appreciated

regards
Marco








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